1. Field of the Invention
The present invention generally relates to optical beam scanning apparatuses, and more particularly to an optical beam scanning apparatus which performs a focal point adjustment over an entire image forming area by using an electrooptic lens which can vary a focal distance under a feedback control.
2. Description of Related Art
In an image forming apparatus such as a digital copy machine, a laser printer or the like, an image is formed on a surface (scanning surface) being scanned by projecting a light beam emitted by a light source which light beam is intensity modulated by an image signal. Before the light beam is projected onto the scanning surface, the scanning surface is uniformly charged, and then the light beam is projected onto the scanning surface by being deflected toward the scanning surface by a deflecting apparatus.
Such an image forming apparatuses has been suggested in Japanese Laid-Open Patent Application No.2-293809. In this image forming apparatus, a distortion of an image formed on the scanning surface is corrected by using an electrooptic lens which varies a focal point of an image forming system by using a driving voltage supplied thereto.
In the above-mentioned image forming apparatus, there is a problem in that an image forming point of the light beam deviates from a point on the scanning surface because of variation of a focal point of the electrooptic lens due to a temperature change in the image forming apparatus. The variation of a focal point may occur due to self-heat generation while the electrooptic lens is operated. Additionally, the image forming point may deviate during an extended period of use because characteristics of the electrooptic lens may change due to aging of the lens. Further, there is another problem in that a variation of a focal distance does not exactly follow a waveform of a driving voltage supplied to the electrooptic lens due to a response delay of the electrooptic lens during an operation or due to a hysteretic characteristic of the electrooptic lens.